1. Field of the Invention
This invention relates to a ring-shaped resonator type semiconductor laser device.
2. Description of the Prior Art
Semiconductor laser devices are laser oscillators which are composed of a Fabry-P/e/ rot resonator which contains therein an active region functioning as an optical waveguide positioned between the reflecting mirros (i.e., the cleaved facets), and which attain laser oscillation by the cleaved facets. These laser oscillators are advantageous in that the reflecting mirrors can be readily and formed in a parallel manner by the cleavage of growth crystals and that mass-production thereof can be attained since a plurality of device units can be produced by the epitaxial growth of crystal layers on a single semiconductor substrate. However, they are inferior in that light is transmitted through the reflecting mirrors, which causes a limitation of the lowering of the threshold current level. The lowering of the threshold current level is a most important issue not only to improve reliability of semiconductor laser devices, but also to advance optical integrated-circuit technology. For these purposes, an AlGaAs ring-shaped semiconductor has been developed and reported in the Japanese Journal of Applied Physics, vol. 16, No. 8, pp. 1395-1398, August, 1977. However, this ring-shaped semiconductor laser device is designed so that the mesa waveguide is formed in a circular shape by an etching technique, so that light-emission loss at the curved portion of the waveguide and light-scattering loss at the etched rough surface of the waveguide arise, which causes difficulties in obtaining a stable light output versus current characteristics. Moreover, the ring-shaped semiconductor laser device is inferior in that it is not provided with an optical coupling means by which laser light is released from the resonator. Thus, this conventional ring-shaped semiconductor laser device has not been put to practical use.